I went to my calendar and it has a whole bunch of notifications about protecting my phone. What do I do?

The other day I was watching something on my phone and a pop-up ad came up. I quickly exited out and went to press play but when I did another thing came across my screen that said subscribe or cancel. I accidentally pressed subscribe and I don’t know what it is. I went to my calendar and it has a whole bunch of notifications about protecting my phone. What do I do? How can I figure out what it is and how can I get rid of it?

[Re-Titled by Moderator]